In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and those unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners, containing pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the photoconductor's surface, either exposed or unexposed to light, depending upon the relative electrostatic charges on the photoconductor's surface, development electrode, and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor's surface, pulling the toner from the photoconductor's surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor's surface. A set of fuser rollers or belts, under heat, melts and fixes the toner in the paper, subsequent to transfer, producing the printed image.
The electrostatic printing process, therefore, comprises an intricate and ongoing series of steps in which the photoconductor's surface is charged and discharged as the printing takes place. In addition, during the process, various charges are formed on the photoconductor's surface, the toner and the paper surface to enable the printing process to take place. Having the appropriate charges in the appropriate places at the appropriate times is critical to making the process work.
After the image is transferred to the paper or other recording medium, it goes to the fuser where the paper is moved through a nip where it is heated and pressed. This melts the thermoplastic portion of the toner, causing it to bond with the fibers of the paper, thereby fixing the image onto the paper or recording medium. While this is an effective way of fixing the toner image on the paper's surface, it carries with it some problems. Specifically, various types of copy media, such as bond paper and tracing paper, contain significant amounts of moisture. During the passage of this paper through the fusing area, the moisture is heated and evaporates. The steam vapor then escapes into other portions of the printer creating the potential for rust and corrosion, which can inhibit machine performance and useful life. The steam can also condense and form puddles in entrapment areas, such as on the surface of the back-up or pressure roller in the fuser. When it does so, it is carried around to the fuser nip, reducing the coefficient of friction between the back-up roller, the paper and the fuser belt. Since in a desktop printer, the back-up roller, through friction, rotates the fuser belt, this reduction in the coefficient of friction causes the paper to slip. This slippage delays the arrival of the paper at the exit sensor, registering as a paper-feed failure, causing the machine to stop. In another scenario, the slippage of the belt, caused by moisture in the fuser area, causes the paper to not enter the fuse nip thereby producing a fuser jam. In both cases, the printer ceases operation, requiring that the operator clear and restart it, delaying completion of the printing project underway.
The problems caused by moisture are particularly acute where the printer utilizes a fuser belt, rather than a fuser roll, especially one that is not self-driven, but rather is driven by friction between the belt, the paper and the back-up roller (which is driven). In this commonly used apparatus, when moisture condenses on the back-up roller, it wets the fuser nip and the fuser belt. This can result in slippage of the paper which delays arrival of the paper at the exit sensor, causing the printer to stop. This requires the operator to clear the paper path and restart the printer to complete the print job. Another problem caused by the presence of moisture is the result of back-up roller/fuser belt slippage. Such slippage can cause a paper bubble, as the paper enters the fuser nip, which not only can result in a paper jam, but can also cause the paper to rub against fuser surfaces, smearing the unfixed toner. These problems are collectively referred to herein as "fuser stalls."
It is clear, for several reasons, that effective removal of moisture, created by the fusing process, from the back-up roller in the belt fuser is very important. The present development describes an effective way to accomplish this goal. Although the prior art recognizes that the production of moisture by the fusing process is undesirable, there are few methods suggested for combating this problem and those methods which have been suggested have significant drawbacks.
U.S. Pat. No. 5,722,026, Goto, et al., issued Feb. 24, 1998, describes a back-up roller which incorporates an elastic layer and a surface layer (fluororesin plus high-friction resin) on an iron or aluminum mandrel. This disclosure does not address in any way the issue of water condensation caused by the fuser in the electrostatic printing process. This patent suggests, at column 1, lines 65-67, that there is a relationship between decreased back-up roller diameter and increased thermal efficiency of the fuser system.
U.S. Pat. No. 4,348,579, Namba, issued Sep. 7, 1982, describes a fuser roller with ribs and reinforcing inserts, which allows the roller to have thin walls for efficient heat transfer, while still providing sufficient strength to withstand the pressure applied in the fusing nip. This patent does not deal with the structure of the back-up roller or with the problem of moisture accumulation caused by the fusing process.
U.S. Pat. No. 5,223,902, Chodak, et al., issued Jun. 29, 1993, describes a moisture collection and removal system for a fuser. The fuser involved does not use a back-up or pressure roller, but rather forms a fusing nip between the fuser roller and a pad biased against the fuser roller. In this system, moisture condenses and falls by gravity into a collection area.
U.S. Pat. No. 4,822,978, Morris, et al., issued Apr. 18, 1989, describes a fuser apparatus which utilizes a low mass fuser roller and a flexible web to keep sheets of paper in biased contact with the fuser roller. The web contains perforations which allow accumulated moisture to escape from the fuser system, the moisture can then be wiped from the outer surface of the web. There is no back-up roller utilized in this system and no structure is given for the wiping mechanism.
U.S. Pat. No. 4,645,327, Kimura, issued Feb. 24, 1987, describes an electrophotographic apparatus which prevents condensation of moisture on the photoconductor's surface. This patent also describes (see column 10, lines 31 et. seq.) a wiper comprised of an aluminum shaft having layers of felt and/or urethane sponge to wipe moisture off the photoreceptor drum. Such structures are generally not effective in dealing with the moisture problem since they tend to absorb water, become saturated, and then feed water back onto the surface of the drum.
U.S. Pat. No. 5,307,133, Koshimizu, et al., issued Apr. 26, 1994, addresses the problem of moisture condensation on the fuser apparatus by incorporating a fan into the printer to eliminate water vapor in the air. This is an indirect way of dealing with addressing the problem which is not as effective as directly addressing the issue by preventing moisture accumulation on the back-up roller.
U.S. Pat. No. 5,091,752, Okada, issued Feb. 25, 1992, addresses the moisture condensation issue by incorporating a heat-insulating surface layer on the back-up roller.
Concurrently-filed U.S. patent application Ser. No. 09/491,610, Belt Fuser Wiper, Burdick, et al., describes the use of a high surface energy material to wipe and remove moisture which condenses on the surface of the back-up roller in the fusing system.
It has now been found that moisture accumulation on the back-up roller can be reduced by utilizing a back-up roller having reduced thermal mass, particularly a roller which comprises an inner cylindrical metal core and an outer hollow cylindrical metal shell surrounding the core, and having a plurality of metal ribs running lengthwise between the core and the shell. The void spaces in this roller reduce the thermal mass of the roller, allowing it to more quickly achieve a temperature comparable to that of the fuser belt, thereby reducing the amount of moisture which condenses on its surface. Such a roller is preferably made by extrusion. This approach effectively reduces the formation of moisture on the back-up roller, thereby eliminating fuser stalls and corrosion of parts; it achieves these ends effectively, inexpensively, and in a manner suited for the small spaces available in a desktop printer context.